New technologies in power conversion have allowed switching frequencies to increase from around 100 kHz to above 20 MHz. See, for example, "Resonant Switches: A Unified Approach To Improving Performances Of Switching Converters", by K. Liu and F. C. Lee, INTELEC 84, IEEE Publication 84 CH 2073-5, pp. 344-359. Also see, "Resonant Switches - Topologies And Characteristics", by K. Liu et al, IEEE PESC Record, pp. 106-116 (1985). Several practical converters have been described with switching frequencies up to 22 MHz. See, for example, the following articles.
"Zero-Voltage-Switched Quasi-Resonant Buck And Flyback Converts - Experimental Results At 10 MHz", W. A. Tabisz, P. Gradzki and F. C. Lee, IEEE Power Electronic Specialists Conference, Blacksburg, Va., pp. 404-413 (June 21-26, 1987).
"Zero-Voltage-Switching Technique In High-Frequency Off-Line Converters", M. M. Jovanovic, W. A. Tabisz and F. C. Lee, Applied Power Electronics Conference Proceedings, New Orleans, La. (February 1988).
"A Resonant DC-To-DC Converter Operating At 22 MHz", Applied Power Electronics Conference Proceedings, New Orleans, La. (February 1988).
U.S. Pat. No. 4,720,667 (Lee et al) relates to zero-current-switched quasi-resonant converters, whereas U.S. Pat. No. 4,720,668 (Lee et al) relates to zero-voltage-switched quasi-resonant converters. At the same time, there are several co-pending applications of interest. In particular, U.S. patent application Ser. No. 856,775 (Lee et al) now U.S. Pat. No. 4,785,387 relates to resonant converters with secondary side resonance; U.S. patent application Ser. No. 99,965 (Tabisz et al) relates to DC-to-DC converters using multi-resonant switches; U.S. patent application Ser. No. 99,952 (Jovanovic et al) relates to off-line zero-voltage-switched quasi-resonant converters; and U.S. patent application filed on even date herewith and entitled "Zero-Voltage-Switched Multi-Resonant Converters Including The Buck And Forward Type" relates to buck and forward multi-resonant converters.
It has been demonstrated that good efficiency can be achieved at these high switching frequencies and considerable size reduction can be expected. However, the full benefits of higher switching frequency cannot be realized unless a high bandwidth, rugged control is used.
Closed-loop regulation of the output voltage of a quasi-resonant converter can be achieved by feedback of the output voltage. Analysis of each of the quasi-resonant power stages shows their small signal characteristics to be similar to their pulse-width-modulated (PWM) counterparts. A compensation network can, therefore, be designed in a similar manner to PWM converter compensation to meet the requirements of closed-loop performance.
A commonly used, single-loop control scheme for a buck quasi-resonant converter (QRC) power stage employs a two-pole, two-zero compensation network to provide high low-frequency gain and improved phase margin at the cross-over frequency. The output filter capacitor suitable for filtering the high switching frequency waveforms of a QRC usually has a very low equivalent series resistance (ESR). The zero due to this ESR usually occurs above the switching frequency and does not improve the phase margin.
In single-loop control, the system includes an error amplifier in which the output is fed to a voltage-controlled oscillator (VCO). Switch drive circuitry for controlling the converter switch is driven by the output of the VCO. In this way, the output of the error amplifier controls the VCO. The switch drive circuitry converts the output signal of the VCO into a constant on-time pulse.
This single-loop control has several disadvantages:
The VCO requires a considerable number of components.
The effective bandwidth of the control is limited by noises considerations: a VCO can be sensitive at higher frequencies; and a large amount of ripple is transmitted through a high-gain amplifier.
The undamped buck QRC and the boost and buck boost QRCs can be very difficult to compensate. Bandwidth and good closed-loop performance must be traded off against stability.
Boost and buck boost converters are especially difficult to control since they have a right-half plane zero in the control-to-output transfer function. This zero places severe limitations on the bandwidth of the control loop and, therefore, good closed-loop performance is difficult to achieve.
The control loop for the single-loop system is quite difficult to design due to an undamped output filter. To achieve transient responses comparable to those with current-sense frequency control (CSFC)control, the system has to be designed close to instability. This explains the oscillatory nature of the transient responses. Single-loop control is quite sensitive to input and parameter variations and becomes unstable at different operating points.
There is thus a need for a control scheme for high-frequency quasi-resonant converters that provides a rugged system with excellent transient response and noise immunity. The present invention is directed toward filling that need.